The present invention relates to a process for recovering molybdenum-99 produced by fission from an irradiated uranium alloy target. More specifically, it relates to obtaining molybdenum-99 for producing technetium-99m generators for medical uses.
Among the radioisotopes used in nuclear medicine, .sup.99m Tc occupies a preponderant position. However, this radioisotope has a very short period (t.sub.1/2 =6 h) and therefore it is supplied to hospitals in the form of a .sup.99m Tc generator charged with .sup.99 Mo, which is the parent element having a longer period (t.sub.1/2 =66 h).
During the decay of .sup.99 Mo, .sup.99 Tc is obtained, which accumulates in the generator and can selectively be eluted therefrom by an isotonic solution, because .sup.99 Mo remains fixed to the alumina under these conditions. Following ageing, fresh doses of .sup.99 Tc can be recovered by the same procedure. The molybdenum-99 absorption capacity of the alumina columns is limited, so that obtaining .sup.99m Tc doses with a high volume activity requires the absorption on the alumina of molybenum with a high .sup.99 Mo content.
Therefore the processes of producing .sup.99 Mo by neutron activation of natural molybdenum leading to a molybdenum of low specific activity have been abandoned for fission .sup.99 Mo production methods in accordance with the reaction: EQU .sup.235 U (n,f) .sup.99 Mo
whose fission yield is 6.1%.
As the thus produced molybdenum-99 is intended for medical applications it is necessary to carefully choose the nature of the target exposed to irradiation, the irradiation conditions and the purification method, which must have high performance characteristics and take account of the following:
(a) the significant safety problems linked with the presence in the target of certain fission products, such as .sup.131 I, .sup.132 Te and .sup.133 Xe, whose chemistry is difficult to control; PA0 (b) the need to obtain a high degree of purity of the .sup.99 Mo required for medical uses; PA0 (c) the need to obtain a .sup.99 Mo recovery efficiency as close as possible to 100%, and PA0 (d) the need of using a minimum performance time due to the relatively short period of .sup.99 Mo. PA0 (a) dissolving the irradiated uranium alloy target in sulphuric acid, PA0 (b) separating the iodine and tellurium present in the thus obtained solution, PA0 (c) oxidizing the molybdenum-99 present in the Mo(VI) solution, and PA0 (d) extracting the thus oxidized molybdenum-99 with a hydroxamic acid of formula: ##STR2## in which R is a radical chosen from the group including straight or branched-chain alkyl radicals, the phenyl radical, phenyl radicals substituted by at least one alkyl radical, arylalkyl radicals and the cyclohexyl radical and R' is a hydrogen atom or an alkyl radical. PA0 n-tetradecanohydroxamic acid of formula: EQU n--C.sub.12 H.sub.15 --CH.sub.2 --CO NHOH PA0 tri-n-butyl-acetohydroxamic acid of formula: ##STR4## tri-i-butyl-acetohydroxamic acid of formula: ##STR5## tri-n-pentyl-acetohydroxamic acid of formula: ##STR6## 2,2'-dimethyl-dodecano hydroxamic acid of formula: ##STR7## 2,2'-diethyl-decano hydroxamic acid of formula: ##STR8## 2,2'-dipropyl-octano hydroxamic acid of formula: ##STR9## 2,2',4-triethyl-octano hydroxamic acid of formula: ##STR10## n-dodecano hydroxamic acid of formula: EQU n--C.sub.10 H.sub.21 --CH.sub.2 --CO NHOH PA0 .alpha.,.alpha.'-dipropyl-phenyl aceto hydroxamic acid of formula: ##STR11## .alpha.,.alpha.'-dimethyl-S(2,4,6-trimethyl-phenyl)-propano hydroxamic acid of formula: ##STR12## .alpha.,.alpha.'-diethyl-S(p-tolyl)-propano hydroxamic acid of formula: ##STR13## .alpha.,.alpha.'-di-n-propyl-cyclohexyl aceto hydroxamic acid of formula: ##STR14##
Hitherto the targets used have either been uranium oxide targets, or uranium alloy targets, such as U-A1.
One of the methods used for recovering molybdenum-99 from targets is to dissolve the target in hydrochloric acid and then extract the molybdenum-99 from the solution obtained using di-(2-ethylhexyl)-phosphoric acid, or by liquid-liquid extraction, or by extraction chromatography.
This procedure suffers from certain disadvantages. Firstly, certain fission products, such as radioactive iodine can be given off in gaseous form at certain stages of the process. Moreover, di-(2-ethylhexyl)-phosphoric acid has a considerable affinity for uranium VI, which leads to the co-extraction of uranium VI and molybdenum-99.
Another known process consists of dissolving an irradiated uranium oxide target in a sulphuric acid solution containing hydrogen peroxide, separating the molybdenum-99 (VI) on a column containing a silver-doped activated carbon and recovering the .sup.99 Mo by elution by means of a soda solution (cf. U.S. Pat. No. 3,940,318).
However, this procedure suffers from the disadvantage of leading to the volatilization of the radioactive iodine due to the presence of the hydrogen peroxide and providing no way of separating the tellurium-132.
Moreover, French Pat. No. 2 344 499 discloses a process for recovering molybdenum-99 from irradiated uranium-aluminium alloy targets. According to this process, the irradiated target is dissolved in a sodium solution, which makes it possible to dissolve the molybdenum-99, whilst maintaining insoluble the uranium and certain fission products. Following filtration, to the filtrate is added an iodine reducing agent, an acid solution (H.sub.2 SO.sub.4 or HCl), then zinc or metallic aluminium and the pH of the solution to 1 or 2M in SCN.sup.- ions, so as to convert the molybdenum into [Mo(SCN).sub.6 ].sup.3-, which is then extracted with di-sec-butyl ether.
This procedure makes it possible to maintain the radioactive iodine in iodide form in solution, but it does not solve the problems caused by the conditioning of these active solutions, particularly the conditioning of tellurium-132. Moreover, it leads to the production of effluents, which are highly concentrated in radiolysis-sensitive SCN.sup.- ions, which can thus be converted into cyanide ions, whilst also involving the use of a dangerous solvent, namely di-sec-butyl ether, whose treatment in the form of contaminated organic effluents is impracticable.
Thus, none of the known processes makes it possible to easily satisfy the aforementioned requirements regarding the problems of safety, purity of the molybdenum-99 and the use of an overall minimum performance time for the process.